Booleans (e.g. Union) are strange!!!

[msynth]

Hi,

My field of work is FE and CFD analysis, for this I use FreeCAD to create geometries which are to some extend complicated and are composed of many smaller parts.
For FE and CFD analysis, normally I create a union of all smaller parts and then export it as Brep or any other format which is acceptable for the preprocessor. The problem occurs in cases, when many parts intersect in one specific point. In this place normally it is not possible to create the mesh because the fused parts act as separated or at least it seems for me so.
I have attached 6 snapshots from three different geometries, for the first case it wasn’t possible to create the mesh at all, for the second it was possible but the quality of mesh wasn't good at all because although geometries are fused but their border lines still exist. And for the third the quality was as desired.
I have done the same with CATIA but, i didn't have this problem.
Could you please have a look to them?

Thanks in advance,
msynth

[wmayer]

for the first case it wasn’t possible to create the mesh at all, for the second it was possible but the quality of mesh wasn't good at all

How did you create the meshes? With FreeCAD or another application? And why wasn't it possible for the first one to mesh it? Your screen shot shows the mesh, doesn't it?

[msynth]

I created the mesh with Salome.

Sorry you are right the screenshots show the mesh but unfortunatelly there is a small mistak; actually "Mesh 3 - Quality good" should be "Mesh 1 - Meshing not possible" and vise versa.

I attached the Geom file too.

[wmayer]

With the mesher from FreeCAD which uses the same library as Salome I got a result. You can go to Mesh > Create mesh from shape. As deviation you can leave the default but for the maximum edge length you should choose 5.0. BTW, this produces a surface mesh not a volume mesh.

[msynth]

I also got the results.
I think the point is that after applying boolean operations (union), we need a function to union faces which lay on a common surface. This function could be embedded in the Union operation or be separated, because sometimes these lines and surfaces are needed.
Salome Geom Modules has such a function in its repair operations.
Repair > Remove Extra Edges > Check the box "union faces, laying on common surface". After checking this box all extra edges and surfaces will be removed. This functionality is great because after applying it the fusion looks very clear and the Mesh quality increases dramatically. As Salome Geom Module is based on OCC Kernel, Probably these repairing possibilities should be available in the OCC Kernel, but i am not sure.
Comparing to Salome, FreeCAD is a very good CAD tool. But there are some functionalities in Salome (such as Repair and Measures), which are very handy. It would be great to have them in FreeCAD, but I don't know how easy it is?
I should also mention that Salome is a very good preprocessor but not a good CAD tool. Therefore linking salome and freecad can be a vital step toward a complete opensource CAD/CAE process.

Regards,
msynth

[wmayer]

AFAIK this cleaning function is part of Salome but not of OCC. Recently I read a comment in the OCC forum about this issue.

For the 0.13 I'll check if we can integrate this functionality because many people complain about that. In most cases it is only out of aesthetic reasons but the meshing is a good argument to have it.